Differences in Radiocesium Export in River Systems 1 and 5 Years After the Fukushima Daiichi Nuclear Power Plant Accident

Studies on the migration of radiocesium, 134Cs and 137Cs, deposited on the ground surface in the Tohoku and Kanto areas are important for understanding their external doses and effects on the ecosystems of freshwater and coastal environments. This chapter focuses on the spatiotemporal variations in radiocesium in river systems with a wide range of accumulation in catchments. Research has been conducted on eight river catchments with different characteristics and radiocesium inventories. We investigated the variations in the radioactivity of 134Cs and 137Cs and their existing forms from 2011 to 2016. The monitoring results indicated that the migration behavior of radiocesium from the watersheds, which were highly contaminated, to the Abukuma and Niida Rivers could be divided into two periods: (1) July 2011 to April 2012 and (2) August 2012 to June 2016. However, the migration in the Natsui River, which had a less-contaminated watershed, differed from that of Abukuma and Niida Rivers, indicating wide range of particulate radiocesium at lower turbidity. These results reflect the differences in the existing forms of radiocesium and a decline in the entry of radiocesium to rivers from catchments at both time intervals.